1. Technical Field
The invention relates to an electro-acoustical device and, more particularly, to a horn loudspeaker particularly suited for reproducing low frequency audible sound at high output levels.
2. Description of the Problem
The reproduction of sound in a compressible medium, such as air, particularly at high intensity levels, poses a number of challenges. One problem is the low efficiency exhibited by compression drivers in transferring energy to air. Conceptually this lack of efficiency is modeled as an impedance problem. Air exhibits an impedance which is low and highly non-resistive in character. The problem of the impedance mismatch has been addressed by use of structures to increase the resistive component of the impedance seen by a compression driver transducer for a given volume of air. A baffle supporting a line array of compression drivers is one such structure. An alternative structure is the horn. A horn is, in effect, an acoustic transformer, and provides an output performance equivalence to a driving unit having a large area diaphragm using a transducer with a relatively small area diaphragm while minimizing cone/diaphragm resonance issues that exist with direct radiator devices. The horn renders radiation impedance seen at the diaphragm increasingly resistive for a given volume velocity of air with the result that increasing power is radiated at a given input power. A horn achieves these results by restricting movement of the air, in other words, movement of the compression driver produces greater local pressure changes than would otherwise occur.
Increasing the acoustic power output from most horn designs has required increasing diaphragm piston travel in order to achieve the required volume velocity of air. Piston travel has been an important limiting factor relating to the amount of power that could be delivered to the horn. This had been seen as limiting the energy that could be introduced to a horn in a given frequency range.
In U.S. patent application Ser. No. 10/649,040, filed 27 Aug. 2003, now issued as U.S. Pat. No. 7,454,030, which is expressly incorporated herein by reference, the present inventor proposed a horn incorporating a plurality of transducers operating in the same frequency range. The transducers were ported to the horn with the ports being distributed along a portion of the propagation axis of the horn. The same drive signal, differentiated only by a phase delay, is supplied each transducer. This allowed the sound wave propagating along the horn to be reinforced in a cascade. By folding the horn, a high volume, low frequency, sound-source was built into a relatively small, energy efficient, package. This package was portable enough to be moved and suitable for open air use resulted from this arrangement. The problem of limited piston travel was addressed in part by dividing the work among a plurality of transducers and adding energy progressively, that is, in a cascade fashion lengthwise along a section of the horn. However, the patent gave no particular guidance relating to spacing between ports from transducer pre-load chambers into the horn or to calibrating the intensity of the output from each transducer. It was sufficient that operation of the transducers produce an output which matched the phase of the wave as it propagated past each port and, implicitly, operated at their power limits without incurring excessive distortion effects in order to maximize the power transferred to the acoustic output.
U.S. patent application Ser. No. 11/362,933, filed 27 Feb. 2006, and now issued as U.S. Pat. No. 7,760,899, is also expressly incorporated herein by reference. In the '933 application the present inventor proposed a modification to the development of the '040 application in which the spacing between output ports was progressively increased along a horn. This arrangement improved Q (directivity).
Other horn designs are known which position transducers at locations spaced from the throat or apex of the horn including horns in which transducers are differentially spaced from a horn “apex” toward the mouth. Such horns have been referred to variously as “multiple entry” horns, “coentrant” horns and “unity summation aperture” horns. These horns position compression drivers at stepped distances from a horn apex. However, all of the compression drivers for a particular frequency range are grouped at a particular spacing from the apex with the highest frequency device(s) being located at the apex and devices suited for lower bandwidths located progressively closer to the horn mouth. U.S. Pat. No. 6,411,718 to Danley et al. exemplifies such devices, showing application to a conical, or more particularly, a pyramid shaped horn.